Control of airway surface liquid (ASL) volume is vital for pulmonary defense against inhaled[unreadable] pathogens/toxicants. Deficits in ASL volume produce airways obstruction and airways infection, reflecting[unreadable] the absence of periciliary liquid (PCL) volume and adhesion of dehydrated mucus to airway surfaces. Much[unreadable] is known about the ion transport processes that control transepithelial ion fluxes, but there are little or no[unreadable] data describing how these processes are coordinately regulated to adjust the mass of salt and, hence, water[unreadable] on airway surfaces in the ranges required for health. Studies of patients with genetic lung diseases, e.g.,[unreadable] cystic fibrosis, have suggested that regulation of both the CFTR and ENaC channels are vital for this[unreadable] process. More recently, a number of clues have suggested a role for nucleotides (NTs) and nucleosides[unreadable] (NSs) in ASL in regulating the balance between Na+ absorption and Cl- secretion to generate ASL volume[unreadable] homeostasis. Indeed, we hypothesize that 1) ASL [NT+NS]s are so critical for ASL volume regulation that in[unreadable] their absence, airway epithelia revert to a purely Na+-absorbing state and deplete all ASL from airway[unreadable] surfaces; and 2) the volume of ASL is proportional to the rate of ATP release (JATP) onto airway surfaces.[unreadable] To test these hypotheses and generate a comprehensive description of ASL volume homeostasis, we[unreadable] propose three Specific Aims: 1) Aim 1 - measure JATP and extracellular NT+NS metabolism to develop a[unreadable] mathematical model that will integrate ASL NT+NS concentrations with a biophysical model of ion transport[unreadable] to describe the regulation of ASL volume homeostasis; 2) Aim 2 - test in human bronchial epithelial (HBE)[unreadable] cultures the requirement for NTs and NSs in the acute regulation of ASL volume homeostasis and the[unreadable] mechanisms that mediate these regulatory processes; and 3) Aim 3 - test the requirement for NT+NS in[unreadable] controlling ASL volume in mutant mouse models in vivo. Relevance to Public Health: Accurate quantitative[unreadable] knowledge of the factors that control ASL homeostasis, i.e., the 'hydration' of airway surfaces, will aid in[unreadable] elucidation of the pathogenesis of major human airways diseases, e.g., COPD, CF, and asthma, and will[unreadable] provide insights into novel therapeutic mechanisms to hydrate airway surfaces and hence, restore normal[unreadable] host defense.